OCTG such as tubing and casing used in the excavation of oil wells for exploration of crude oil and gas oil are usually connected to each other by threaded joints. In the past, the depth of oil wells was 2,000-3,000 meters, but in deep oil wells such as recent offshore oil fields, it may reach 8,000-10,000 meters.
In their environment of use, threaded joints for connecting OCTG are subjected to axial tensile forces caused by the weight of the OCTG and the threaded joints themselves, the combination of internal and external pressures, and geothermal heat. Accordingly, they need to be able to maintain airtightness without undergoing damage even in such a severe environment.
A typical threaded joint used for connecting OCTG has a pin-box structure in which a pin has a male thread formed on the end portion of an oil well pipe and a box has a female thread formed on the inner surface of a threaded connecting member (a coupling). An unthreaded metal contact portion is formed at the end of the male thread of the pin and at the base of the female thread of the box. By inserting one end of an oil well pipe into a threaded connecting member and fastening the male thread and the female thread to each other, the unthreaded metal contact portions of the pin and the box are made to contact each other to form a metal-to-metal seal portion which ensures airtightness.
During the process of lowering tubing or casing into an oil well, due to various problems, it is sometimes necessary to loosen a joint which has been once fastened, to lift the pipes out of the oil well, to refasten them, and then relower them. API (American Petroleum Institute) requires galling resistance such that unrecoverable severe seizing referred to as galling does not occur and airtightness is maintained even if fastening (makeup) and loosening (breakout) are repeated ten times for a joint for tubing or three times for a joint for casing.
At the time of fastening, in order to increase galling resistance and airtightness, a viscous liquid lubricant which contains heavy metal powders and which is referred to as “compound grease” is applied to the contact surfaces (namely, the threaded portions and the unthreaded metal contact portions) of a threaded joint. Such a compound grease is specified by API Bulletin 5A2.
In the past, it has been proposed to carry out various types of surface treatment such as nitriding, various types of plating including zinc plating and composite plating, and phosphate chemical conversion treatment on the contact surfaces of a threaded joint to form one or more layers in order to increase the retention of compound grease and improve sliding properties. However, as described below, the use of compound grease poses the threat of harmful effects on the environment and humans.
Compound grease contains large amounts of powders of heavy metals such as zinc, lead, and copper. When fastening a threaded joint, grease which has been applied is washed off or overflows to the exterior surface, and there is the possibility of its producing harmful effects on the environment and especially on sea life, particularly due to harmful heavy metals such as lead. In addition, the process of applying compound grease worsens the working environment, and there is also a concern of its having harmful effects on humans.
In recent years, as a result of the enactment in 1998 of the OSPAR Convention (Oslo-Paris Convention) for preventing ocean pollution in the Northeast Atlantic, strict restrictions concerning the global environment are becoming more numerous, and in some regions, the use of compound grease is already in the process of restriction. Accordingly, in order to avoid harmful effects on the environment and humans in the excavation of gas wells and oil wells, a demand has developed for threaded joints which can exhibit excellent galling resistance without using compound grease.
As a threaded joint which can be used for connecting OCTG without application of compound grease, the present inventors proposed in JP 2002-173692A a threaded joint for steel pipes having a viscous liquid or semisolid lubricating coating formed thereon, and in JP 2004-53013 a threaded joint for steel pipes in which tackiness of the threaded joint surface, which is a drawback of a viscous liquid or semisolid lubricating coating, is suppressed by covering the lubricating coating with an upper lubricating layer which may be based on a certain powder or oxide wax so as to minimize the adhesion of foreign matter such as dust, sand, and debris.